Data Encoding Reading Assignment Stallings Chapter 2 pp

Data Encoding Reading Assignment : Stallings Chapter 2, pp. 45 - 60, Chapter 4, pp. 96 - 101, 103 - 105, 107 - 111 • Terminology – Data • entities that convey meaning – Signals • electromagnetic representation of data – Signaling • the act of propagating the signal along a suitable medium – Transmission • the communication of data by the propagation and processing of signals. – Encoding • converting analog or digital data to analog or digital signals

• Data – analog • continuous values on some interval • examples – audio / acoustic » sound waves perceived by human » 20 Hz - 20 k Hz » above 700 Hz -- slight intelligibility added – video » amount of illumination at each point » time varying analog signal » horizontal and vertical retrace » 483 horizontal lines 30 scans / sec » interlace : 241. 5 lines 60 scans / sec to avoid flickering – digital • e. g. , text or character strings • ASCII – 7 bits + parity bit

• Signals – analog • continuously varying electromagnetic wave • medium : twisted pair, coaxial cable, fiber optic cable, atmosphere • examples – speech spectrum : 20 Hz to 20 k Hz » telephone transmitter : 300 to 3400 Hz » Either high frequencies of speech are cut off, or » for higher fidelity, convert acoustic signal into an electromagnetic signal over 300 3400 Hz. – video signal » horizontal blanking pulse » horizontal sync pulse : to maintain transmitter-receiver synchronization » vertical blanking pulse – Digital • e. g. , high voltage for 1, low voltage for 0 • See chapter 4.

– modem (modulator-demodulator) • digital analog (carrier frequency) digital – codec (coder-decoder) • analog digital analog • methods of transmission – analog • amplifier – digital • repeater : regenerates digital signal – digital transmission preferred • cost dropping for integrated circuits (not as much for analog equipment) • Effects of noise, etc. are not cumulative (due to the use of repeaters). • effective use of large capacity of medium • security and privacy

• Transmission impairments – Attenuation • The signal strength falls off logarithmically with distance for guided medium. • Attenuation is an increasing function of frequency. – Decibel notation : 10 log 10 ( P 2 / P 1 ) • Considerations – The transmitter generates a signal as strong as possible without causing distortion. – The received signal must be well above noise level. – Delay distortion • The signal velocity over a guided medium varies with frequency. • critical for digital data – Intersymbol interference » Some of the signal components of one bit position spill over into other bit positions.

– Noise • unwanted signals inserted between transmitter and receiver • types – thermal noise » thermal agitation of electrons in a conductor » across entire frequency spectrum » cannot be eliminated – intermodulation noise » generated by two or more frequencies sharing the same transmission medium » f 1 + f 2 or f 1 - f 2 – crosstalk » electrical coupling between nearby twisted pair » unwanted signals picked up by microwave antennas » same order of magnitude as thermal noise – impulse noise » noncontinuous, irregular pulses/spikes, short duration, relatively high amplitude » electromagnetic disturbances, lightning » affects digital signals most

• Data encoding techniques – analog signaling • carrier signal • analog medium : usually band limited • modulation – the process of encoding source data onto a carrier signal with frequency fc. – input : modulating signal (baseband signal) – transmitted : modulated signal – four possibilities • analog/digital data analog/digital signal – digital data, digital signals • terminology – signal element – signaling rate / baud rate / modulation rate – unipolar signaling » all voltage levels positive or negative – polar signaling » voltage levels both positive and negative

• Facts – An increase in data rate increases bit error rate. – An increase in S/N ratio decreases bit error rate. – An increase in bandwidth allows an increase in data rate. • factors affecting receiver in interpreting digital signals – – timing of each bit (clocking) signal level for each bit position S/N ratio, data rate, bandwidth encoding scheme • desired properties in encoding scheme – signal spectrum » lack of direct-current (dc) component » lack of high-frequency components, i. e. , lower bandwidth » transmitted power concentrated in the middle of the transmission bandwidth – synchronization mechanism for clocking – error detection capability – noise immunity

• Nonreturn to Zero (NRZ) coding techniques – Nonreturn-to-Zero-Level (NRZ-L) » 1 : constant positive voltage » 0 : constant negative voltage – NRZI » nonreturn to zero, invert on ones » A transition at the beginning of a bit time denotes a binary 1; no transition indicates a binary 0. – Limitations » presence of dc component » lack of synchronization capability – used for signal input, digital magnetic recording • differential encoding – signal decoded by comparing the polarity of adjacent signal elements – advantages : easier to detect a transition – E. g. , if the leads of a twisted pair is inverted, NRZI still gives correct results, but NRZL does not.

• Biphase coding techniques – at least one transition per bit time – Manchester code » 1 : low-to-high transition » 0 : high-to-low transition » used in Ethernet – Differential Manchester code » 1 : absence of transition at beginning of bit period » 0 : presence of transition at beginning » differential coding used » used in token ring – mid-bit transition : clocking – advantages » synchronization » no dc component » error detection : absence of an expected transition error • Modulation rate (D) – rate at which signal elements are generated. – Data rate (R) : bits per second – e. g. , Manchester encoding » R <= D <= 2 R

– digital data, analog signals • carrier frequency fc • phone lines : 300 - 3400 Hz • modulation techniques – Amplitude-shift keying (ASK) » binary 1 : A cos( 2 fc t ) » binary 0 : 0 » susceptible to sudden gain change » 1200 bps on voice-grade phone lines » ASK is used in transmission over optical fiber. – Frequency-shift keying (FSK) » binary 1 : A cos( 2 f 1 t ) » binary 0 : A cos( 2 f 2 t ) » f 1 , f 2 : fc » used in 1200 bps modems, highfrequency radio transmission – Phase-shift keying (PSK) » data represented by shifting phase of carrier signal » A cos( 2 fc t + s )

– Two-phase differential PSK » binary 1 : A cos( 2 fc t + ) » binary 0 : A cos( 2 fc t ) – Quadrature phase-shift keying (QPSK) » binary 11 : A cos( 2 fc t + /4 ) » binary 10 : A cos( 2 fc t + 3 /4 ) » binary 00 : A cos( 2 fc t + 5 /4 ) » binary 01 : A cos( 2 fc t + 7 /4 ) – Combining PSK and ASK » 9600 bps modem, 12 phase angles, 2 amplitude values, on 2400 bps baud line